Electrolytics and voltage ratings?

Discussion in 'Tube Audio' started by nj pheonix, Sep 14, 2017.

  1. nj pheonix

    nj pheonix AK Subscriber Subscriber

    New Jersey
    I completely get that cap has to be rated to voltages it see (with some room for surges and such.
    I was reading in another thread that cap (E) should be run between 60-75% of their rating to stay formed properly.
    The reason I bring it up a have a 500v multi section can (reasonably new maybe 2-3 years old) left over from an earlier project. Size and C values would work well in new project but actual voltages are all 275v and under.
    So based on information I read, this cap would fail prematurely.
    Now the old ones ,we see, go 20, 30,40, sometimes 50 years. If I had confidence this would go 15 years (assuming it wasn't defective in the first place). I be happy with that. Should I be concerned?


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  2. whoaru99

    whoaru99 Epic Member

    Last bit I was reading from Cornell Dubilier had some comments on voltage rating/derating -

    Rated DC Voltage
    Rated DC voltage is the nominal voltage marked on the capaci-
    tor, and it is the maximum peak voltage including ripple volt-
    age that may be applied continuously between the terminals
    and over the rated temperature range. Higher rated voltage ca-
    pacitors may be substituted for lower rated voltage capacitors
    as long as case size, DF, and ESR ratings are also compatible.
    Voltage Derating
    Voltage derating is expressed as the percentage that the applied
    voltage is less than rated voltage, e.g., a 450 V capacitor operat-
    ing at 400 V would have 11% voltage derating.

    Aluminum electrolytic capacitors made with formation voltages
    at least 35% higher than rated voltage and with rated tempera-
    tures of 85 ºC or higher, don’t require much voltage derating. In
    applications operating at less than 45 ºC no derating is needed,
    and with up to 75 ºC, 10% is sufficient. For higher temperatures
    and with high ripple current, 15% or 20% is appropriate. Since
    operating life continues to increase for further derating, military
    and space applications use 50% voltage derating.

    Photoflash capacitors may be used at full rated voltage at nor-
    mal room temperatures because they are designed for such
    duty. Strobe capacitors benefit from at least 10% voltage de-
    rating because their continuous operation makes them run hot.

    At face value, if voltage derating of 50% is common in military and space applications for longer life seems like it would be good for you and me too; if one is looking for every last ounce of prevention. :dunno:

    I think some of the comments I've see about using capacitors at lower voltage are in reference to running caps at a small fraction of rated voltage. To me, 50%, is not in that small fraction category.
    Last edited: Sep 14, 2017
  3. nj pheonix

    nj pheonix AK Subscriber Subscriber

    New Jersey
    Thanks for sharing.
    My gut told me it'd probably be fine. Until I read it I the other thread, I wouldn't have given it a second thought.
    After reading it and thinking about it after, I figured it'd be worth getting some extra input.
  4. triode17

    triode17 Well-Known Member

    I have never heard that the cap. would fail prematurely. An electrolytic cap. forms to what voltage is applied. If you apply 275v and less to the 500v cap, it would simply reform to that set of voltages. If you leave a cap. uncharged for years, it will form back to zero volts. That's why one should never just plug an old amp. in to see if it works. There becomes a race between formation voltage and temperature. If voltage wins out, all is well. But if temp. rises faster than the cap. can reform..........BANG!! or sisssssss.
    Morgan Jones wrote in the book Valve Amplifiers, that a cap. run down to 89% of its rating, will last twice as long as full voltage. So by you using the caps. at 275v or less, it should last for a long time.
  5. gadget73

    gadget73 junk junkie Subscriber

    Southern NJ
    right, at some point that 500v cap won't be able to take 500v without a slow ramp-up to reform it. It'll become basically a 300v cap but if you aren't going to pull it and use it in something else that really doesn't matter any.

    If you do want to do that, tickle the voltage in with a nice drop resistor. 10K and overnight should do it just fine. You can work out the leakage current across the resistor and that will tell you if it took it. As long as the resistor is sufficiently large to keep the current down it won't blow up on you.

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